1. Field of the Invention
The subject invention pertains to the preparation of thermoplastic sandwich structures wherein facing sheets or skins are applied to a fiber-reinforced or non-reinforced thermoplastic core in a rapid and cost-effective manner. More particularly, the invention relates to the fabrication of thermoplastic honeycomb sandwich structures wherein facing sheets or skins having a layer of thermoplastic film are applied to thermoplastic honeycomb core substantially simultaneously with application of the pressure bonding.
2. Description of the Related Art
Honeycomb sandwich panels and foam core sandwich panels are used in the transportation, aerospace and recreation industries where a planar or modestly curvilinear structure which is also exceptionally light is needed. In copending application Ser. No. 07/531,184 now U.S. Pat. No. 5,139,596, the entire disclosure of which is incorporated herein, by reference thereto, a continuous method for the production of thermoplastic honeycomb is disclosed which should enable increased utilization of these structural components.
Following production of honeycomb or foam core by whatever method, the finished core is sawn or slit into planar segments of the required thickness and typically, a heat-curable thermosetting film adhesive, for example an epoxy resin is used to bond the skin, which is generally a film, or a fiber reinforced prepreg or laminate, to the core. The core assembly, with facing sheets and film adhesives, is heated while pressure is applied to the assembly. Following a curing time which is generally several hours long, the assembled sandwich panel may be cut into appropriate size for its intended application.
The typical processes of bonding the honeycomb or foam core to the facing sheets as practiced by prior art processes are time consuming and often utilize relatively expensive film adhesives whose adhesion, viscosity, and cure cycles must often be tailored for each specific application. Typically, if either a thermoplastic, thermoset or other type of facing is desired, then a heated platen press is used with press stops. The heat for fusion or for advancing the thermoset adhesive is transferred from the platen through the skin material (thereby, sometimes limiting the thickness of the skin material) to the underlying core material, the adhesive fused or cured, whereupon the sandwich structure is then cooled under pressure before removing the assembled and bonded sandwich structure. When bonding facing materials with this aforementioned method to a thermoplastic core material such as a thermoplatic honeycomb or thermoplastic foam core, the thermoplastic core material has a tendency to collapse while the skin is bonded to the surface. This is due to excessive heat transfer through the skin to the core, thus raising the temperature of certain portions of the core beyond the T.sub.g (or softening point) of the thermoplastic core material with a resulting distortion or collapsing of the thermoplastic core during the skin bonding process. This is particularly a problem when one tries to bond a skin material with a T.sub.g nearly the same as or higher than that of the core material itself. The method taught by Hudson (U.S. Pat. No. 4,249,976) is typical of the art practiced today in sandwich panel construction wherein a skin material is adhesively bonded to the core wherein the adhesive layer is a thermoplastic adhesive (see column 3, lines 23-30) or for that matter, more typically, an epoxy thermoset film adhesive, a method which has been in use for decades. The Hudson method comprises; a) layering a sheet of thermoplastic (or thermoset for that matter) onto the core; b) laying a skin thereon; c) heating the assembly to soften (or cure) and bond the skin to the core; and d) cooling the assembly (see column 4, lines 27-29 and column 4, lines 64-column 5 line 2). Westlake (U.S. Pat. No. 4,902,365), Zaima (U.S. Pat. No. 5,022,943) and Wolf (U.S. Pat. No. 4,496,024) all teach methods of fabrication which are similar to that of Hudson's. The common factor to all of the aforementioned patents is that essentially the entire sandwich panel assembly is heated to the bonding temperature while under pressure. If a thermoplastic core were used in any of the aforementioned methods, the collapse or distortion of the core would occur if the temperature required for bonding exceeds the T.sub.g of the honeycomb material.
Umeda (U.S. Pat. No. 5,037,498) and Landi (U.S. Pat. No. 5,039,567) describe alternative methods for heating the entire honeycomb structure. Both, however, describe a multiple step method which requires that the heat required for bonding be applied down through the facing material while simultaneously applying pressure through the heat transfer device, typically a platen. This pressure is maintained until adequate heat has been transferred to the underlying honeycomb material through the skin material and is sufficient to cause either (in the case of Landi), the melting of the cell edges of the honeycomb material, whereupon, the pressure device is then rapidly cooled before removal of the pressure, or (in the case of Umeda), curing of a thermoset adhesive attached to the honeycomb cell edge.
In the case of Landi, the core cell edges are first deformed prior to bonding a skin in place (column 5, lines 39-47), thus, most likely, increasing the thermal mass of the cell edges relative to the thermal mass of the facing material and thereby, possibly reducing the probability of honeycomb collapse. Umeda is, essentially, just a continuous, stepwise progression of any of the methods described above (column 3, line 63 to column 4, line 47).
Additionally, all of the aforementioned methods, using a thermoplastic or thermoset facing material, require the use of a release substrate between the heating/pressure device and the facing material during the bonding cycle.
Additional prior art of note concerns the continuous lamination of plastic materials. Kajikawa (U.S. Pat. No. 4,872,930) (column 4, lines 20-37) and Williams (U.S. Pat. No. 3,769,129) (column 2 lines 9-14) describe the continuous application of thermoplastic materials in layer form upon some other entity. They are typical in that they describe the heating of the thermoplastic material in its entirety, usually from the outside surface to the inner surface, prior to the consolidation and cooling steps.
Finally, the known art of bonding facing materials to honeycomb for structural panels has generally been limited to either all thermoset materials, combinations of metal facings and thermoset cores, thermoplastic facings glued with thermoset adhesives or lower melting thermoplastic adhesives to thermoset and more recently, thermoplastic honeycombs or multiple step processing of similar thermoplastic materials. The use of facings made of material the same or higher T.sub.g as the honeycomb core or foam core materials themselves, or film layers of the same or higher T.sub.g than the facings and/or core materials has not been done before on a continuous basis nor has it been taught elsewhere how one would overcome the problems of core collapse that the use of these same materials would cause.